Use -PdisableFirebase to build a non-Firebase build

[ahmedre]

The two flags, -PdisableCrashlytics and -PdisableFirebase are the same,
so consolidate to one flag to avoid confusion.

[github-actions]

OLD: app-madani-debug.apk (signature: V1, V2)
NEW: app-madani-debug.apk (signature: V1, V2)

          │          compressed          │         uncompressed         
          ├───────────┬───────────┬──────┼───────────┬───────────┬──────
 APK      │ old       │ new       │ diff │ old       │ new       │ diff 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
      dex │  24.1 MiB │  24.1 MiB │  0 B │  74.7 MiB │  74.7 MiB │  0 B 
     arsc │   2.7 MiB │   2.7 MiB │  0 B │   2.7 MiB │   2.7 MiB │  0 B 
 manifest │   5.8 KiB │   5.8 KiB │  0 B │    28 KiB │    28 KiB │  0 B 
      res │   1.7 MiB │   1.7 MiB │  0 B │     2 MiB │     2 MiB │  0 B 
   native │  18.8 KiB │  18.8 KiB │  0 B │  36.5 KiB │  36.5 KiB │  0 B 
    asset │ 404.2 KiB │ 404.2 KiB │  0 B │ 678.6 KiB │ 678.6 KiB │  0 B 
    other │   218 KiB │   218 KiB │  0 B │ 464.3 KiB │ 464.3 KiB │  0 B 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
    total │  29.2 MiB │  29.2 MiB │  0 B │  80.5 MiB │  80.5 MiB │  0 B