Adjustments after moving MediaStore inside APEX.

The "core_platform" SDK technically includes non-stable APIs, so we
need to shift to "core_current" instead.

While we're here, tidy up some older @removed APIs that never shipped
and replace HexEncoding with a local implementation.

Since MediaProvider itself is free to use @hide APIs from its own
framework code in MediaStore.java, we can include all sources
from "framework-mediaprovider-sources" directly.

Also since the "framework-annotations" filegroup in Android.bp is
a very limited subset of leaf source code, it can't directly
reference Intent, so adjust the javadoc so we can drop the "import."

Bug: 144247087
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I198d84a755b5e270a19b41328f6a6e152112c40a