Remove redirect_get_content and add read-only variant

Turns out when I added this flag initially I forgot the
is_fixed_read_only property. This has inadvertently caused the
AndroidManifest.xml to retain the key android:featureFlag against the
TrampolineActivity.

When I change it to read-only, it actually compiles the property out
of the AndroidManifest and retains one or the other. Unfortunately
flags which incorrectly set this property can't be updated, they have
to be abandoned. So let's use redirect_get_content_ro to fix this
mishap.

http://ag/32017425 is rolling the existing flag back, so will submit
this CL once that has landed.

Bug: 377771195
Flag: com.android.documentsui.flags.redirect_get_content_ro
Test: atest com.android.documentsui.picker.TrampolineActivityTest
Change-Id: I4e41c0ab87ebc59272faef5351fefddbf5873654