Bump minSdk to 21

Test: presubmit
Bug: 322508920
Bug: 329997430
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1448a452b523404d442ebd02fd5d2c0241ce8609)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4b893fefe52be428fcb67a3f257eef9c38368e14)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:2268fda36b609bda9dd1990d7ba18696ba0d9b6f)
Merged-In: I080e1ab31139379423e74e44ff3fa76b36becbb3
Change-Id: I080e1ab31139379423e74e44ff3fa76b36becbb3
1 file changed