Remove CryptKeeper

Since Android 10, new devices have been required to use FBE (File Based
Encryption) instead of FDE (Full Disk Encryption).  FDE support was
already removed from system components such as init and vold.
Therefore, the CryptKeeper activity is no longer needed.  Remove it.

Note: this CL only removes CryptKeeper itself, i.e. the activity which
runs at boot time on devices that are using FDE or are being encrypted
with FDE in-place.  Later CLs will remove FDE-specific code from the
Settings app.

Bug: 208476087
Change-Id: I4aaf795e8cee1ff3cdd55a41c975273c8faeefa9