Use safe-mmio crate for 8250 UART driver.

This takes care of working around the Rust volatile write issue on
aarch64 and also lets us remove some unsafe code here.

Test: atest vmbase_example.integration_test
Change-Id: I73e2d16ed55b134d5851f94eb226a1e9104da495

diff --git a/libs/libvmbase/Android.bp b/libs/libvmbase/Android.bp
index 2d0294e..c02466e 100644
--- a/libs/libvmbase/Android.bp
+++ b/libs/libvmbase/Android.bp
@@ -83,6 +83,7 @@
         "liblibfdt_nostd",
         "liblog_rust_nostd",
         "libonce_cell_nostd",
+        "libsafe_mmio",
         "libspin_nostd",
         "libstatic_assertions",
         "libthiserror_nostd",